Hardware wouldn't add LLC-SNAP when skb->data_len is 0, which causes
Tx hang, so add a check to drop this kind of packet.
Tested-by: Bo Jiao <bo.jiao@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>